Reported is a time-resolved infrared and optical kinetics investigation of the transient species CH3C(O)Mn(CO)(4) (I-Mn) generated by flash photolysis of the acetyl manganese pentacarbonyl complex CH3C(O)Mn(CO)(5) (A(Mn)) in cyclohexane and in tetrahydrofuran. Activation parameters were determined for CO trapping of I-Mn to regenerate A(Mn) (rate = k(CO) [CO][I-Mn]) as well as the methyl migration pathway to form methylmanganese pentacarbonyl CH3Mn(CO)5 (M-Mn) (rate = k(M)[I-Mn]). These values were DeltaH(not equalCo) = 31 +/- 1 kJ mol(-1), DeltaS(not equalCo) = -64 +/- 3 J mol(-1) K-1, DeltaH(M)(not equal) = 35 +/- 1 kJ mol(-1), and DeltaS(M)(not equal) = -111 +/- 3 J mol(-1) K-1. Substantially different activation parameters were found for the methyl migration kinetics of I-Mn in THF solutions where DeltaH(M)(not equal) = 68 +/- 4 kJ mol(-1) and DeltaS(M)(not equal) = 10 10 J mol(-1) K-1, consistent with the earlier conclusion (Boese, W. T.; Ford, P. C. J. Am. Chem. Soc. 1995, 117, 8381-8391) that the composition of I-Mn is different in these two media. The possible isotope effect on k(M) was also evaluated by studying the intermediates generated from flash photolysis of CD3C(O)Mn(CO)(5) in cyclohexane, but this was found to be nearly negligible (k(M)(h)/k(M)(d) (298 K) = 0.97 +/- 0.05, DeltaH(not equal) (d)(M) = 37 +/- 4 kJ mol(-1), and AS Md = -104 +/- 12 J mol(-1) K-1), The relevance to the migratory insertion mechanism of CH3Mn(CO)5, a model for catalytic carbonylations, is discussed.